In the assembly of electronic components, the solder bump interconnection method was developed to eliminate the expense, unreliability, and low productivity associated with manually wire bonding integrated circuit chips onto carrier or printed circuit substrates. The method utilizes solder bumps deposited on solder-wettable metal terminations on the chip and a matching pattern of solder-wettable terminations on the substrate. In the case of an upside down integrated circuit (flip chip), the flip chip is aligned to the substrate and all the joints are made simultaneously by melting the solder. Typically, the solder bumps are placed on the integrated circuit terminals while the chip is still in wafer form.
To join the integrated circuit to the substrate, a flux, generally a no-clean, low residue flux, is placed on the substrate as a temporary adhesive to hold the integrated circuit in place. The assembly is subjected to a solder melting thermal cycle in an oven or furnace thereby soldering the chip to the substrate. The surface tension of the solder aids to self align the chip to the substrate terminals. After this reflow step, removing the flux residue from under the chip is difficult and requires sophisticated cleaning regimes. This is due, in part, to the close proximity of the chip to the substrate which typically is about 0.001 to 0.006 in. (25.4 to 150 .mu.m). Therefore with conventional techniques the flux residues are generally left in the space between the chip and the substrate. For this reason, the residues must be inert to prevent subsequent corrosion of the assembly.
The next step in the assembly process is to encapsulate the chip which permits the use of polymeric substrates in flip chip assembly process. Encapsulation can result in significant improvements in the fatigue life of the solder bumps as compared to an unencapsulated flip chip assembly. The technique of underfill encapsulation has gained considerable acceptance by the electronics industry and the materials of choice for underfill encapsulation have been epoxies. Polymeric substrate materials have higher coefficients of thermal expansion than that of a silicon chip, but the expansion coefficients of the underfill encapsulant epoxies can be adjusted with the addition of ceramic fillers to reduce the level of thermal stress that develops between the substrate and the encapsulant.
Most underfill encapsulation procedures involve dispensing liquid encapsulants onto one or more edges of the flip chip assembly. Capillary action draws the encapsulant material through the minute gap between the chip and the substrate. As is apparent, this underfill process can be quite slow due to the small dimensions. The gap must be completely filled and, after the epoxy is cured, be free of voids in order to provide adequate protection for the device and reduce fatigue of the solder joints. The flux residues remaining in the gap reduce the adhesive and cohesive strengths of the underfill encapsulating adhesive.
As is apparent, the multi-step assembly process has a number of deficiencies. Furthermore, as the size of chips increases, the limiting effect of capillary action becomes more pronounced and renders the encapsulation procedure even more time consuming. As a result, it is expected that there will be a greater tendency for the epoxy polymer to separate from the ceramic filler during application. In addition, there will be more void spaces.
In an attempt to alleviate these problems associated with flip chip assembly, the industry has employed polymer flux compositions. Prior art polymer flux compositions suitable for assembling electronic components and particularly flip chips generally comprise an elaborate mixture comprising a thermosetting or thermoplastic resin, a flux activator that is generally halogenated, and a chemically protected curing agent which can also function as a fluxing agent. See, for example, U.S. Pat. Nos. 5,088,189, 5,128,746, 5,136,365, 5,167,729, and 5,417,771, and EP 0 620 077 A1.
Prior art flux compositions containing thermosetting resins are undesirable due to their high viscosity and short shelf life which is typically less than one week. Moreover, conventional multi-component fluxing compositions are intrinsically not self-crosslinking. They require resins, such as epoxy resins, for crosslinking thereby further limiting the shelf life or pot life of the material and decreasing flux activity substantially. In addition, chemical protection of the carboxylic acid in the fluxing agent was essential to achieving stability and preventing premature reactions. This results in an acid that is functioning at much less than its full strength with the metal oxides.
As is apparent, the art is in search of more efficient chip assembly techniques that can consistently produce an essentially voidless underfill encapsulation. In particular, there is a need for a soldering flux that can remove oxides and promote soldering without use of ionic or halogen-containing flux activators and that can serve as an adhesive to provide improved bonding at electrical interconnections.